A basic heat pipe comprises a closed or sealed envelope or a chamber containing a liquid-transporting wick and a working fluid capable of having both a liquid phase and a vapor phase within a desired range of operating temperatures. When one portion of the chamber is exposed to a relatively high temperature it functions as an evaporator section. The working fluid is vaporized in the evaporator section causing a slight pressure increase forcing the vapor through an adiabatic flow channel to a relatively lower temperature section of the chamber defined as a condenser section. The vapor is condensed in the condenser section and returned through the liquid-transporting wick to the evaporator section by capillary pumping action.
Because it operates on the principle of phase changes rather than on the principles of conduction or convection, a heat pipe is theoretically capable of transferring heat at a much higher rate than conventional heat transfer systems. Nevertheless, a number of difficulties have been experienced in attempting to use heat pipes for certain applications.
For example, when the wick is made of a capillary material such as a fine-pore wire mesh, the rate of fluid mass flow and consequently heat transfer is limited due to the high pressure drop encountered by the fluid as it flows through the wire mesh. To eliminate this pressure drop, permit increased fluid flow rates and increase heat transfer rates or heat transport capacities, improved heat pipes such as, e.g., pedestal-artery type heat pipes have been fashioned.
In a pedestal-artery type heat pipe, a fluid-conducting wire mesh artery is supported by a wire mesh stem in fluid communication with a wicking medium or fine circumferential grooves disposed on the inner periphery of the heat pipe wall. The fluid-conducting artery is generally designed to promote automatic priming or filling. Once filled, the artery characteristically has a pressure drop equivalent to a round tube and allows relatively high heat transport capacities to be achieved.
In the absence of gravity (e.g., in space), any size artery of this type can theoretically prime. However, most heat pipes suitable for use in space applications must pass a ground (gravity) test before the heat pipe can be used. In the presence of gravity, artery priming is governed by design factors limiting heat pipe transport capacities to only thousands of watt-inches (heat transport rate times distance). However, analysts have estimated that future heat pipe transport capacities in the range of millons of watt-inches may be required thereby necessitating a new approach to artery design.
Recently, a monogroove heat pipe has been developed permitting relatively high heat transport capacities without impacting heat transfer efficiency. It combines the advantages of simple construction and large liquid and vapor areas, with the high heat transfer coefficients of circumferential wall grooves. The basic monogroove design contains two large axial channels, one for vapor and one for liquid. A small slot separates the channels thereby creating a high capillary pressure differential which, coupled with the minimized flow resistance of the two separate channels, results in the high axial heat transport capacity. The high evaporation and condensation film coefficients are provided separately by circumferential wall grooves in the vapor channel without interferring with the overall heat transport capability of the heat pipe.
The thermal performance of the monogroove heat pipe is deleteriously affected by two major factors. For example, continuous liquid flow between the axial liquid channel (artery) and the circumferential wall grooves in the vapor channel must be assured. This continunity must be maintained with groove menisci realistically depressed to reflect maximum heat flux conditions. Unfortunately, this continunity may not be readily maintained in actual use because liquid in the liquid channel has a tendency to boil, due to its proximity to the evaporator section, thereby disrupting flow in the axial slot.
In addition to boiling problems, the monogroove heat pipe is limited by the slot connecting the liquid and vapor channels. To promote high liquid flow rates from the liquid channel to the circumferential grooves, a wide slot should exist. To promote maximum surface tension pumping in the axial slot, the slot should be narrow (i.e., to produce a small meniscus radius). These two competing factors cause the slot width to be set at some intermediate value which neither optimizes meniscus pumping ability nor slot pressure drop.
These two problems do not exist in the present invention. The heat pipe transport system discussed herein utilizes a unique slot cover to maximize axial slot surface tension pumping while permitting a wide axial slot and attendant low slot viscous pressure drops. In addition, steps have been taken to minimize thermal interaction between the liquid channel (artery) and slot, and the heat source.